1. Field of Invention
The present invention relates to a mode of plasma assisted substrate processing in which a gaseous species is ionized, dissociated, or otherwise modified in a plasma and the modified species or a component thereof is caused to strike the substrate.
2. Prior Art
Over the past several decades, plasma assisted substrate processing has found increasing acceptance in a variety of industries, particularly within the semiconductor industry where the fundamental process steps include reactive ion etching (RIE), plasma assisted chemical vapor deposition (CVD), sputtering, reactive sputtering, and ion assisted physical vapor deposition (PVD). Processes that are currently employed in these industries generally operate in a continuous mode. That is, the only parameter that is varied, or modulated, with time, or temporally, is the radio frequency (RF) energy, which inherently varies at the RF excitation signal period.
In fact, temporal modulation of process parameters on a time scale that is small relative to the process time yet large compared with the RF period is limited to a few select areas of research. Temporal modulation of gaseous species is employed in atomic layer epitaxy (ALE) and pulsed jet epitaxy (PJE), a derivative of ALE, and more recently the temporal modulation of RF power has demonstrated improvements to selectivity purportedly as a result of electron temperature control.
The temporal modulation of RF power that has been studied and used simply involved the temporal modulation of the RF amplitude. It is conventional when treating periodic functions (of time) to represent these functions in discrete Fourier space, viz.
                              u          ⁡                      (            t            )                          =                              ∑                          n              =                              -                                  N                  2                                                                    N              2                                ⁢                                    A              n                        ⁢                          ⅇ                                                ⅈω                  n                                ⁢                t                                                                        (        1        )            where An is the Fourier amplitude and ωn is the angular frequency (=2πnf0). In general, the applied RF signal takes the above form wherein the Fourier harmonic amplitudes An are independent of time. However, pulsed RF application has generally included An=An(t).
The concept of pulsing gases in combination with pulsing the RF power in the performance of etching, deposition and related processes is disclosed, for example, in Heinecke et al., U.S. Pat. No. 4,824,690. This patent proposes a plasma reactor that allows for introducing different gases alternatingly, in a pulsating manner, into a processing chamber at an alternation rate which is on a time scale consistent with processing gas exchange rate and concurrently pulsing the RF power coincident with the beginning of each gas alternation cycle. The RF power is pulsed between off and approximately 60 kW with a pulse width of 50 to 500 msec (a duty cycle of approximately 0.1 to 1%) at the start of each gas introduction pulse. Heinecke et al., U.S. Pat. No. 4,935,661 discloses that the gas exchange rate can be improved by pulsing the gas admission under high pressure.
Although both of the above-cited Heinecke et al. patents are directed to deposition processes, they mention application of the disclosed technique to other processes such as etching.
As alluded to earlier herein, the temporal modulation of gaseous species spans several disciplines, including atomic layer epitaxy (ALE), chemical vapor deposition (CVD), molecular beam epitaxy (MBE), pulsed jet epitaxy (PJE), pulsed molecular beams, pulsed gas injection and pulsed gas valves. In the patent literature, three types of pulsed gas injection have been identified: pulsed jet; run/vent; and pulsed “train”. Pulsed jet injection, as the name implies, is performed by controlling a gas injector in order to inject gas in the form of a series of discrete pulses. In a run/vent configuration, one or more gas streams are alternately switched between (“run”) chamber injection or (“vent”) exhaust. The exhaust or vent system must mimic the chamber conditions (i.e. pressure, etc.). Pulsed “train” gas injection uses a continuously flowing carrier gas into which different gases are cyclically introduced by periodic injection into the primary carrier gas, thus producing a gas “train” In order to minimize gas diffusion between adjacent species, the gas pulsing operates at a sufficiently low duty cycle to permit carrier gas to act as a diffusion barrier. In other words, when two or more different processing gasses are to be injected in alternation, flow of both or all processing gasses is blocked for a short period prior to injection of either gas into the carrier stream, thus separating successive doses of the two or more processing gasses from one another. These three types of pulsed gas injection are described, for example, in Blakeslee, U.S. Pat. No. 3,721,583; Boucher, U.S. Pat. No. 3,979,235; Suntola et al., U.S. Pat. No. 4,058,430; and Suntola et al., U.S. Pat. No. 4,413,022.
Often the exchange of gases during known ALE processes occurs under low pressure injection and over gas exchange periods sufficiently longer than the reactor gas residence time, i.e., several minutes to tens of minutes.
PJE is an extension of ALE and operates with supersonic jets at high rates. Several papers by Ozeki et al. correlate the use of high speed jets with gas exchange rates having periods as low as 100 msec. These papers include: New approach to the atomic layer epitaxy of GaAs using a fast gas stream, Appl. Phys. Lett., Vol. 53, p16, (1988); Growth of GaAs and AlAs thin films by a new atomic layer epitaxy technique, Thin solid films, Vol. 174 (1989); and Pulsed jet epitaxy of III–V compounds, J. Crys. Growth, Vol. 107 (1991). Further, Eres et al., U.S. Pat. No. 5,164,040, describes a PJE technique that employs an array of pulsed supersonic jets supplied by a source reservoir providing processing gas at a delivery pressure ranging from a few Torr to 200 Psi. Additionally, a plurality of jets that can be pulsed with any variation of frequency and phasing relative to each other.
Similar to ALE, the PJE technique promotes selective epitaxy and can produce highly uniform depositions.
Lastly, studies of pulsed molecular beams suggest the possibility of producing pulsed molecular beams with short temporal pulse widths and high repetition rates. Typically, the literature reports that pulsed molecular beams have been generated using high speed EM valves, some similar to car fuel injector valves, and piezo-electric devices. By way of example, technology of this type is described in the following papers: Gentry & Giese, Resolved single-quantum rotational excitation in HD+He collisions . . . , J. Chem. Phys., Vol. 67, p 11 (1977); Balle et al., A new method for observing the rotational spectra of weak molecular complexes . . . , J. Chem. Phys., Vol. 72, p2 (1979); Bassi et al., Pulsed molecular beam source, Rev. Sci. Instrum., Vol. 52, p1 (1981), Cross et al., High repetition rate pulsed nozzle beam source, Rev. Sci. Instrum., Vol. 53, p 38 (1982) and Andresen et al., Characteristics of a piezo-electric pulsed nozzle beam, Rev. Sci. Instrum., Vol 56, p 11 (1985). Results from time-of-flight (TOF) and UV laser induced fluorescence measurements indicate that pulse times as short as 50 μsec and repetition rates as high as 1000 Hz are achievable.
Pulsing the RF power to a plasma has been utilized primarily to enhance selectivity and/or uniformity, as well as to affect charging damage. The focus of prior work has been to temporally modulate the RF power between off and on, and thereby achieve improved selectivity and uniformity by tuning the pulse width and pulse repetition rate (PRR). In particular, it has been proposed in the art to temporally modulate the RF power in order to control the products of dissociation in the plasma and, in turn, control the reactants of the etch or deposition chemistry. In essence, pulsed modulation of the RF power reduces the electron temperature in an average sense over a pulse cycle, and hence directly affects the time average of the electron energy distribution. The degree, or rate, of dissociation and ionization of molecules within the plasma is proportional to the number of electrons and the collision cross-section, the latter being dependent upon the electron energy. Subsequently, one can control the chemical reactants for substrate processing by controlling the electron energy distribution within the plasma.
Hou et al., U.S. Pat. No. 3,677,799, describes using pulsed RF power to control boron coating deposition. Gorin et al., U.S. Pat. No. 4,263,088, discloses use of emission spectroscopy to determine the end of an etch process whereupon the RF power is switched from a continuous mode to a pulsed mode. Several other patents suggest pulsing the RF power on a time scale that is small compared with the time to significantly deplete reactants. See, for example, Engle et al., U.S. Pat. No. 4,401,507 and Ellenberger et al., U.S. Pat. No. 4,500,563. Such pulsing has been shown to enhance etch/deposition selectivity and uniformity.
More recently, the use of RF pulsing to control etch selectivity has been reported. Sugai et al., Diagnostics and control of high-density etch plasmas, Mat. Res. Soc. Symp. Proc., Vol 406, p 15 (1996), describes performance of advanced diagnostics on inductively coupled plasma reactors (ICP). Through variation of the pulse width, amplitude and repetition rate, improvements in the selectivity of SiO2 to Si has been achieved by controlling the relative concentration of CF2 to CF3, CF and F. In fact, Samukawa, in Highly selective and highly anisotropic SiO2 etching in pulse-time modulated ECR plasma, Jpn. J. Appl. Phys., Vol. 33(1), p 2133 (1994), identified a direct relationship between the ratio of CF2 to F and the pulse duration. Moreover, Labelle et al., in Effect of precursors on the properties of pulsed PECVD fluorocarbon thin films, NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Thrust, A Teleconference, (Nov. 6, 1997), reports enhanced CF2 ratios in work on pulsed PECVD fluorocarbon thin films.
In addition to improving selectivity in oxide etch processes, Samukawa et al., in Pulse-time modulated electron cyclotron resonance plasma etching for highly selective . . . , J. Vac. Sci. Technol. B, Vol. 12(6), p3300 (1994), discusses control of the ion energy spectra in polysilicon etching using pulsed RF power. Yeon et al., in Study of particulate formation and its control by a radio frequency power modulation . . . , J. Vac. Sci. Technol. B, Vol. 15(1), p 66 (1997), claims to reduce particulate formation via RF modulation.
Furthermore, Ono et al., in “Selectivity and profile control of poly-Si etching by time modulation bias method”, 1998 Dry Process Symposium (V1–5), p. 141–146, reported improved poly-Si to SiO2 etch selectivity, improved etch anisotropy and suppressed micro-trenching phenomena when pulse modulating the chuck bias.
Lastly, Ohtake et al., in “Reduction of topography-dependent charging damage by the pulse time modulated plasma”, Dry Process Symposium (V-1), p. 97–102, and Matsui et al., in “Effect of pulse modulated plasma on a charge build up of the microscopic structure, 1998 Dry Process Symposium (IV-2), p. 85–90, have investigated pulse modulated plasma with regard to the alleviation of topography dependent charging damage.
The concept of temporally modulating (or pulsing) the gas species or RF power delivered to a plasma is thus known. In fact, as already described, these concepts have been investigated extensively. However, known wafer processing technologies based on inductively coupled plasma (ICP) technology lack independent control of reaction chemistry and ion bombardment. For example, the plasma conditions necessary to produce optimal etch reactants via dissociation of a process gas are not the same as the conditions necessary for delivering optimal ion energy (and ion energy distribution) to the substrate.
Furthermore, known chamber configurations are incapable of making efficient use of reactive gases, or of efficiently removing volatile etch products.